Water cooled nuclear reactors, and in particular nuclear reactors cooled with pressurized water, comprise a core constituted by prismatic assemblies disposed side by side and vertically. Each of the fuel assemblies is constituted by a cluster of parallel fuel rods maintained in a framework.
The framework of the fuel assembly is constituted by a plurality of transverse spacer grids spaced apart along the length of the assembly and maintaining the rods in a regular network in cross-sectional planes of the cluster. The spacer grids are interconnected by longitudinal guide tubes which are substituted for the fuel rods in some of the cells of the spacer grids and fixed to the latter.
The framework of the assembly also comprises two end nozzles connected to the end of the guide tubes whose length exceeds the length of the fuel rods.
One of the nozzles of the assembly, termed the bottom nozzle, rests on the bottom plate of the core of the reactor which is provided with apertures in the region of each of the assemblies to permit the passage of the cooling water of the reactor through the core in the vertically upward direction.
The cooling fluid of the fuel rods passes through the adaptor plate of the bottom nozzle through openings termed water passages which are either of circular shape (with a diameter of about 7 to 10 mm) or oblong (openings which are about 10 mm wide and 15 to 50 mm long). Fragments which may be present in the primary circuit of the reactor are liable to be carried along by the circulating water under pressure and, in the event that their size is small (for example, less than 10 mm), these fragments may pass through the adaptor plate of the bottom nozzle whose water passages have a large section, and may become wedged between the fuel rods and the cells of the first grid, i.e., the spacer grid which is the closest to the bottom nozzle.
These fragments, subjected to the axial and transverse hydraulic forces which are high in this zone, may cause wear of the sheathing of the fuel rod. There is consequently a risk of impairment of the fluid-tightness of this sheathing and an increase in the rate of activity of the primary circuit of the reactor.
Devices have therefore been proposed for filtering the cooling fluid of the reactor either during tests under hot conditions or during the operation of the reactor.
In the first case, the filtering elements may be connected to the bottom core plate and arranged on the latter in the position of the fuel assemblies before loading the core.
In the second case, the filtering elements are associated with the fuel assemblies and located in their lower part. These filtering elements may be formed by structures of sheet metal or metal wires disposed in the bottom nozzle of the assembly for arresting fragments whose size exceeds the largest dimension of the section of the passage between a fuel rod and a cell of the grid.
Such devices may be complicated and result in a relatively large pressure drop in the circulation of the cooling fluid through the fuel assembly.
Further, these devices placed in the bottom nozzle of the assembly may be relatively bulky and constitute an obstacle when loading and unloading the assemblies of the core and when connecting or disconnecting the guide tubes and the bottom nozzle.